FIG. 13 shows a longitudinal sectional view of a prior art and is the combustor containing a fuel nozzle disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-2848. As shown in FIG. 13, a pilot nozzle 300 is provided on a center axis of an inner tube 180 of a combustor 100. A plurality of fuel nozzles 200 which extend substantially in parallel with the pilot nozzle 300 are equally spaced in a circumferential direction around the pilot nozzle 300. Fuel is supplied to the pilot nozzle 300 and fuel nozzles 200. A swirl vane or a swirler 290 is disposed around a rodlike body of the fuel nozzle 200. A plurality of hollow columns 250 which radially and outwardly extend from the sidewall of the fuel nozzle 200 are provided on the fuel nozzle 200. The hollow columns 250 are connected to the fuel nozzle 200. A plurality of injection ports 260 are provided in each hollow column 250 to inject fuel toward a tip end of the fuel nozzle 200. A mixing chamber 150 is formed in the vicinity of the tip end of the fuel nozzle 200, and a pilot combustion chamber 160 is defined by a pre-mixing nozzle 170 in the vicinity of the tip end of the pilot nozzle 300.
Air for combustion that enters the combustor 100 through an air inlet 110 thereof is reversed through about 180° at an inner tube end portion 120 and flows into an air passage 140. A part of the air for combustion is mixed with fuel injected from injection ports 260 of the hollow column 250 and, then flows into the swirler 290 of the fuel nozzle 200. Accordingly, the air for combustion is mainly turned in a circumferential direction, and mixing of the air for combustion and the fuel is promoted. Thus, pre-mixed air is produced in the mixing chamber 150.
The remaining air for combustion flows into the swirler 390 disposed between the pilot nozzle 300 and the pre-mixing nozzle 170. The air for combustion is burnt with fuel injected from the tip end of the pilot nozzle 300, in the pilot combustion chamber 160, to produce a pilot flame. Pre-mixed air mixed with fuel injected form the injection ports 260 of the hollow column 250 is brought into contact with the pilot flame and then is burnt to produce a main flame.
In the combustor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-2848, fuel is injected from the hollow column having a fuel injection port so that the fuel is uniformly mixed with air. In order to enhance a mixing action, increasing the number of injection ports per one hollow column 250 and increasing the number of hollow columns 250 has been considered. However, the number of the hollow columns and the number of injection ports are physically limited and, thus, the enhancement of the mixing action is limited. In general, the occurrence of NOx tends to increase as the ratio of fuel to combustion air is increased, i.e., a hot spot occurs. Therefore, it is preferable that fuel be uniformly mixed with air.
In the pre-mix type combustor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-2848, the spatial density of energy released by combustion is increased when the combustion is carried out in a relatively narrow space. Consequently, combustion vibration occurs. The combustion vibration is associated with a columnar resonance, and is determined by the length, capacity and flow resistance of the combustor. In this case, the concentration of fuel varies due to velocity fluctuations in the pre-mixing nozzle 170 and, then, the combustion vibration, a self-excited vibration phenomenon, occurs. The combustion becomes unstable due to the combustion vibration, and the combustor cannot be driven stably. Therefore, it is necessary to prevent the occurrence of combustion vibration.
Japanese Patent Application No. 2000-220832 discloses a combustor nozzle in which a velocity fluctuation absorbing member is provided in an inlet portion to take air therein so as to prevent the occurrence of the combustion vibration. In this prior art, the velocity fluctuation absorbing member produces a flow resistance to absorb the velocity fluctuation resulting from the combustion vibration, and thus the occurrence of the combustion vibration is prevented.
However, in the combustor disclosed in Japanese Patent Application No. 2000-220832, the air passes through the velocity fluctuation absorbing member positioned in the inlet portion and is reversed by about 180° at an inner tube end portion and, then, flows toward the swirler and the mixing chamber. Namely, in the above-described Japanese Patent Application No. 2000-220832, a distance between the velocity fluctuation absorbing member and the mixing chamber is relatively long. Therefore, there is a possibility that an air turbulence occurred by the velocity fluctuation absorbing member in the inlet portion is decreased in the vicinity of the mixing chamber, or completely disappears in the vicinity of the mixing chamber. The installation of the velocity fluctuation absorbing member of the combustor disclosed in Japanese Patent Application No. 2000-220832 is strictly for the purpose of control of the combustion vibration, and a mixing action resulting from the turbulence is not taken into consideration. Therefore, it is necessary to maintain the turbulence of the airflow when the mixture of fuel and air is enhanced by the turbulence.
In the above-described combustor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-2848, there is a limit to an increase in the number of injection ports because the diameter of the injection port of the hollow column is determined depending on a machining accuracy or a problem of hole clogging. Further, when the number of hollow columns is increased, it is difficult to supply air to the mixing chamber because the hollow columns 250 interrupt the airflow. Therefore, a method for enhancing a mixing action of fuel and air without increasing the number of the hollow columns and the injection ports of the hollow column is demanded.
In the velocity fluctuation absorbing member positioned in the air inlet portion disclosed in Japanese Patent Application No. 2000-220832, it is assumed that the combustion vibration cannot be effectively reduced under the influence of the capacity of air existing between the air inlet portion and a pre-mixer. Accordingly, a more effective combustion vibration reducing structure, which is hardly influenced by the capacity on the upstream side of the pre-mixer, is required.
Therefore, the object of the present invention is to provide a gas turbine combustor in which the occurrence of the combustion vibration is prevented while the mixing action of fuel and air is enhanced.